US President Barack Obama and Chinese President Hu Jintao promised a determined, joint effort to tackle climate change, nuclear disarmament and other global troubles yet emerged from their first full-blown summit Tuesday with scant progress beyond goodwill.After two hours of talks and a separate meeting over dinner the night before, the presidents spoke of moving beyond the divisiveness over human rights, trade and military tensions that have bedeviled relations in past decades."The major challenges of the 21st century, from climate change to nuclear proliferation to economic recovery, are challenges that touch both our nations, and challenges that neither of our nations can solve by acting alone," Obama said, standing with the Chinese leader in the Great Hall of the People.Hu, who heads a collective leadership that often has preferred to go it alone internationally, said: "There are growing global challenges, and countries in today's world have become more and more interdependent."With each of those big issues - from global warming to the Iranian and North Korean nuclear programs - persistent differences bubbled up in the form of indirect barbs during the joint appearance.Stung by new US levies on imports of Chinese-made tires and steel pipes, Hu said he told Obama that given a still struggling global economy both countries "need to oppose and reject protectionism in all its manifestations in an even stronger stand."Obama later called on China to relax controls that keep the Chinese currency relatively weak and thus help fuel exports - something Beijing officials have rejected in recent days. Obama also pointedly raised human rights, saying they are fundamental to all."We do not believe these principles are unique to America, but rather they are universal rights and that they should be available to all peoples, to all ethnic and religious minorities," Obama said in his only nationally televised remarks on the sensitive issue.The mixture of promises and lasting differences underscored how intertwined the superpower United States and rising power China are, and the difficult task Obama faces in managing friction with an authoritarian, sometimes testy Beijing.On his first visit ever to China, Obama said he was mostly striving to better understand China, a geopolitical force on its way to becoming the world's second-largest economy."Our relationship going forward will not be without disagreement or difficulty," Obama said. "But because of our cooperation, both the United States and China are more prosperous and secure."Aside from his meetings with Hu, Obama received a formal welcome. He walked past rows of soldiers in dress uniforms and dined on chicken soup with bean curd, Chinese-style beef steak and roast grouper at a state banquet. He also toured the Forbidden City, the emperors' palace for 400 years, and met the head of China's legislature, a former mayor of Shanghai, the commercial hub where Obama started his three-day stay in China.In a minor advance, the two leaders set a deadline of early next year for resuming an on-again, off-again dialogue on human rights. Charting a new frontier for cooperation, the two agreed to reciprocal visits by the heads of their space programs. Promises were made to step up visits by military leaders to help overcome years of distrust over a Chinese military buildup and US reconnaissance missions in the seas off China.Obama and Hu held their fullest conversation ever on shoring up Afghanistan and Pakistan, US and Chinese officials said. The Obama administration has tried to coax the Chinese leadership to assist in the troubled region. But Beijing has been reluctant, despite growing economic interests in the region and worries that the conflicts could spill across China's borders into restive, heavily Muslim western China.Headway was made on climate change. The two committed their countries - the biggest emitters of the heat-trapping gases causing global warming - to backing a detailed political agreement at next month's climate-change conference in Copenhagen. In their formula, rich countries would commit to reduction targets while developing ones would agree to meet softer goals that would be monitored.Yet the positions were not markedly different from those Beijing and Washington held before Obama's arrival.So it also was with attempts to curb Iran's nuclear program and disarm nuclear-armed North Korea. Though Obama talked of continuing diplomatic efforts on Iran and North Korea, Hu did not endorse the US leader's talk of sterner actions should negotiations falter. Beijing has strong interests in keeping North Korea stable and in maintaining budding energy cooperation with Iran."Iran has an opportunity to present and demonstrate its peaceful intentions, but if it fails to take this opportunity, there will be consequences," the US president said. Hu did not mention consequences.Keeping the differences veiled rather than open was a measure of success of sorts for Obama. With its economy still in trouble, US international prestige still battered and China holding $800 billion in US government debt, Obama came to the Beijing summit with a weaker hand than previous US presidents. That makes the emphasis on practical cooperation all the more needed, Chinese analysts said."The Chinese leadership will not worry too much about the US pressure. In the context of the financial crisis and George W. Bush's legacy on the issues of Iran, North Korea, Afghanistan and Pakistan, the US needs China much more than China needs the US," said Yu Wanli, an America expert at Peking University.At their joint appearance, Hu called on the US to respect China's "core interests" - code for ending support for Taiwan and for the Dalai Lama, in his Tibetan government-in-exile. Obama obliged by saying Tibet was part of China. But he urged China to restart talks with the Dalai Lama's representatives - something Hu did not mention.